1. Field of the Invention
The present invention relates to an optical disk apparatus which reads data in units of code blocks arranged by joining a plurality of error-correcting codes from an optical disk.
2. Description of the Related Art
In the past, in an optical disk apparatus in which information is read from and written to an optical disk, the bit error rate of user data has been improved sufficiently enough for practical use by means of predetermined error correcting codes.
FIG. 8 shows a sector format for an optical disk established by the International Organization for Standardization (ISO). In FIG. 8, only relations between the user data and the error correcting code are shown, and illustrations of an address unit, a gap, a VFO (variable frequency oscillator), a synchronizing signal, and the like are omitted. The optical disk apparatus arranges blocks of user data that are successively input, forms an ECC (Error Correcting Code) block by adding the error correcting code based on LDC (Long Distance Code) to the arranged blocks, and then records the user data on the optical disk specified according to the sector format shown in FIG. 8 by allocating one ECC block to one sector.
The optical disk apparatus forms one ECC block using ten interleave lines. The code size of each interleave line is 120 words (eight bits of data constitutes one word) in which 104 words are allocated for the user data and 16 words are allocated for the error correcting code (parity) using the LDC. The optical disk apparatus sets 120xc3x9710 words for one ECC block as recording words for the optical disk, adds 16 words of control data to the ECC block, and then assigns the resultant ECC block to each sector of the optical disk.
Interleave is applied to data in the direction of the disk. Here, the correcting is one-pass normal correcting. In order to perform more efficient correcting, combined correcting may be considered using a remove flag obtained from information other than coding information, such as servo error, RF signal degradation of data, and out-of-synchronization information.
FIG. 9 shows the ECC block format of a digital versatile disk (DVD). The DVD uses a PRC (product code) as the error correcting code. Random error correcting is performed along with checks for burst errors in accordance with C1 (which is a correction-code direction in the direction of the disk). When the code cannot be corrected, the remove flag is added to C1 code series and then combined correcting is performed in the C2 direction (which is a correction-code direction in which interleave is applied). The correcting can be repeated.
Recently, optical disks and optical disk apparatuses with large capacity and high transfer speed have been desired particularly for use with moving pictures and the like. As a method for increasing the capacity of the optical disk, high-density recording which is obtained by shortening the wavelength of a light source or by increasing the numerical apertures of writing/reading beams of the optical system may be considered. When the numerical aperture of the writing/reading beams of the optical system are increased, it is preferable that the thickness of the substrate of the disk be thinned so that a skew margin and the like can be obtained.
When the disk has a higher density and the thickness of the optical disk becomes thinner, the disk is susceptible to dust, flaws, and the like. Therefore, the error-correcting ability thereof must be increased. Specifically, the error-correcting ability for burst errors must be high.
By setting the code length to be long, the error-correcting ability is increased. By setting the interleave length to be long, the error-correcting ability, particularly for the burst error, is increased. That is, by increasing the size of the ECC block, the error-correcting ability is expected to be increased in a system in which the burst errors are dominant.
When the error correcting is performed using the LDC as the error correcting code, even though the remove information other than coding information (obtained from external information) is used to increase the correcting efficiency, because the external information, such as servo error, RF signal degradation of data, and out-of-synchronization information does not necessarily agree with the data error, the validity of the remove information is sometimes uncertain.
When the error correcting is performed using the PRC as the error correcting code, the C2 correcting ability must be secured in the system in which burst errors are dominant. In a system in which random errors are less influential than burst errors, although less C1 correcting ability is required, the C1 parity becomes important for detecting burst errors. However, the existence of the C1 parity reduces the user data efficiency of the code block. When the user data efficiency is fixed at a certain level, correcting abilities of C1 and C2, that is, parity balance, sensitively affect relations between error characteristics (for example, whether the dominant error is burst errors or random errors) and correcting results.
Shortening of the wavelength of the light source, increasing the numerical aperture of the writing/reading beams in the optical system, and the like enable a code block having 64 Kbytes or more of user data to be recorded in one track of the innermost track of a data region of a 12 cm disk (which is equivalent to a DVD).
In order to increase the correcting ability as much as possible, it is preferable to use as large a code block as possible. However, in RS (Reed Solomon) code employing a common GF(2{circumflex over ( )}8), it is difficult to arrange a code block having 64 Kbytes or more of user data unlike the typical DVD using the PRC.
Although as high an increase as possible in the capacity of the optical disk is desired, significant reduction in the coding efficiency and increase in redundancy are not desired. In addition, as high an increase as possible in the correcting ability is desired. Particularly, an increase in the correcting efficiency for burst errors is desired. Efficient burst-error correcting is desired for securing the random-error correcting ability as well. That is, a correcting method for checking for burst errors and performing remove correction without changing the common LDC format is desired.
Accordingly, it is an object of the present invention to provide an optical disk apparatus and a data reading method which are capable of improving error-correcting ability even though excessive dust, flaws, and the like cause more than expected occurrence of errors in a large data volume of the optical disk.
To this end, according to a first aspect of the present invention, there is provided an optical disk apparatus for reading data written in an optical disk in units of code blocks arranged by joining a plurality of correcting codes. The optical disk apparatus includes a reading unit for reading the code block from the optical disk and an error correcting unit for combining first error correcting processing performed on the code block read by the reading unit with second error correcting processing performed by adding remove information to codes that cannot be corrected during at least the first error correcting processing.
According to a second aspect of the present invention, a data reading method reads data written in an optical disk in units of code blocks arranged by joining a plurality of correcting codes. The data reading method includes an error correcting step of combining first error correcting processing performed on the code block read from the optical disk with second error correcting processing performed by adding remove information to codes that cannot be corrected during at least the first error correcting processing.